PHOENIX — Leading liberals and progressive groups are turning their gaze away from the Democratic primary and toward efforts to unite the left against Donald Trump, framing him as a dangerous and unprecedented candidate who poses an existential threat to the progressive movement and the nation.

Ad campaigns are in the works. There are calls for massive voter mobilizations and screeds from leading donors circulating within liberal circles. Major labor groups are organizing their members on the ground in swing states. Within the Democratic Party apparatus, top elected leaders are beginning to speak out both collectively and individually in ways that reach beyond standard presidential-year posturing. The sense of urgency in some corners of the left is high enough that Massachusetts Sen. Elizabeth Warren, a progressive icon who has gone to great lengths to avoid being part of the Democratic presidential conversation in 2016, hinted on Monday that she might soon get involved with the effort to stop Trump.

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“Ever since early January, when I was door-knocking with our members in Cedar Rapids [Iowa], I heard concerns from our members about what he was saying and doing, and I would say [that] inside our union, our objection to his hate-filled rhetoric has been growing,” said Mary Kay Henry, president of the Service Employees International Union, a 2 million-member labor group that’s supporting Hillary Clinton.

Henry is a signatory to one of the most conspicuous anti-Trump efforts to date, an open letter calling progressives to arms against the billionaire. Signed by leaders of more than 20 leading liberal groups that support Clinton or Bernie Sanders, the letter described Trump in the harshest of terms and called for a massive organizing effort against him.

“This is a five-alarm fire for our democracy. A hate-peddling bigot who openly incites violence is the likely presidential nominee of one of our nation’s two major parties,” reads the letter, signed by leaders of groups as varied as MoveOn, the Sierra Club, NARAL Pro-Choice America and Greenpeace. “It is alarming and dangerous. Donald Trump’s candidacy is a threat to the America we love, and we must respond to him and what he is stoking as such.”

The letter, released Tuesday, served as a public acknowledgment of the heightened apprehension surrounding Trump’s candidacy, as well as a sign that leaders in the party’s left wing are now setting their sights more squarely on Trump than the protracted Democratic nomination battle between Clinton andSanders.

“As more and more progressive groups were with each other in the early primary states and talking about our general organizing, we decided to link our fights and invite others to join us,” explained Henry.

The question of what to do about Trump has gone from a bad joke to a topic of frequent conversation among leading Democratsincluding members of Congress. As Trump has stormed toward the GOP nomination, confounding the Republican establishment and steamrolling veteran officeholders in his path, liberal leaders have come to conclude that the time has come to more seriously confront Trump — whom, they warn, could actually win the White House.

“The message is becoming clearer, more in focus,” said Rep. Xavier Becerra, chairman of the House Democratic Caucus and a Clinton supporter, referring to the Democratic lines of attack against Trump.

Some of the party’s best-known figures are taking the lead on the piecemeal and as-yet-uncoordinated operation to stop Trump. Senate Minority Leader Harry Reid was set to deliver a “major address” on the billionaire real estate mogul and progressives’ response at the liberal Center for American Progress on Thursday.

And Warren, the liberal hero who had to bat away entreaties to run for president in early 2015, told reporters she is getting more involved in the presidential election as she watches “a form of ugliness and extremism that is erupting on the right [that] not only threatens the Republicans, it threatens the entire country," according to a Boston Herald report.

Outside groups and top donors have also pitched in. The AFL-CIO, which hasn’t endorsed a candidate, has started to unleash anti-Trump digital ads and door-knocking in Ohio and Pennsylvania, two Rust Belt swing states where Democrats have been concerned Trump’s economic populism could take hold.

Tom Steyer, the billionaire environmentalist who was the nation’s biggest spender during the 2014 primaries, and who has suggested he’ll give even more in 2016, wrote a Medium post on Monday declaring, “Together, we are proclaiming to the world that Trump is not ours. We will not be silent. We will not stand back and let him put a torch to the ideals and principles that Americans hold dear.”

Many Democrats believe a galvanized anti-Trump movement would serve to unite the party after a contentious primary. Butome fear, it could also prove divisive — perhaps even premature — because the need to concentrate the full force of the left’s resources against Trump could result in calls for Sanders to withdraw from the race before the Vermont senator and his supporters are ready to let go.

People close to Clinton are already making that case implicitly, with the candidate herself at the forefront.

“The sooner I could become your nominee, the more I could begin to turn our attention to the Republicans,” she said in Detroit earlier this month, a week before telling a crowd that results were falling in her favor, but that it’s more important to focus on Trump and his “dangerous path.”

Those calls for unity behind Clinton are still rubbing some leading liberals the wrong way. Sanders-supporting Democracy For America, for example, insists that its efforts to elect the underdog are part and parcel of an attempt to stop Trump.

“The bottom line for us is actually that we’ve had our eyes on Trump for months; it’s one of the reasons we’re so whole-heartedly supporting Bernie Sanders’ campaign,” said DFA’s executive director, Charles Chamberlain, pointing to preliminary general election polling showing Sanders would beat Trump by a larger margin than Clinton would. “As it’s becoming more official Trump will become the nominee, what you’ll see is DFA working harder and harder in highlighting and exposing Donald Trump for who he is: a bigoted, racist demagogue."

“The reality is that Bernie Sanders is an antidote to Donald Trump’s hate,” he added. “Anybody who is going to exclusively go after Trump without continuing to engage in the Democratic primary is making a mistake."

Yet to the former secretary of state, Trump — not Sanders — is now the task at hand.

In West Palm Beach on Tuesday night, after taking a dramatic step toward the nomination, Clinton demonstrated her intention to serve as the leader of the Trump resistance.

“Our commander in chief has to be able to defend her country, not embarrass it,” she said, just hours after her campaign released an Arizona-specific Spanish-language ad aimed squarely at Trump.

Later that night, across the country in Phoenix, it was Sanders who offered to lead the allied liberal forces. Dramatically dialing back his usual stump speech section that highlights his contrasts with Clinton, Sanders proceeded to insteadtake on the night’s big winner on the Republican side: Trump.